1. Field of the Invention
The present invention relates to a power supply circuit in which a switching semiconductor element is provided between a power supply and a load, and more particularly, to an ON failure detecting apparatus that detects an ON failure of a semiconductor element.
2. Description of the Related Art
Generally, in a power supply circuit that includes a semiconductor element, such as a MOSFET or the like, disposed between a power supply and a load, and switches between ON and OFF of the semiconductor element so as to control ON and OFF of a power supply to the load, if an ON failure occurs in the semiconductor element, the semiconductor element enters a state where control is not possible, and thus it is not possible to stop a power supply to the load. At the same time, in a case in which an excess current protecting function assuming that the semiconductor element is normally intercepted is provided, the excess current protecting function is damaged. In this state, if failures such as short-circuiting and grounding of a wiring line at a downstream side of the semiconductor element happen simultaneously, they causes a problem in that the semiconductor element and the wiring line cannot be protected.
Accordingly, in order to prevent an ON failure of a semiconductor element of a power supply circuit in which a semiconductor element is disposed on a high side (power supply side) of a load, the following methods of (a) to (d) have been generally considered.
(a): A fuse is provided on an upstream side (power supply side) of a semiconductor element.
(b): Two semiconductor elements are connected in series to each other, and when there occurred an ON failure in one of the two semiconductor elements, a circuit is intercepted by using the other semiconductor element.
(c): The reliability of a semiconductor element and a control circuit are improved to be so reliable that an occurrence probability of an ON failure could be reduced.
(d): An ON failure detecting circuit and a back up circuit are prepared. When an ON failure occurs in a semiconductor element, failure information is output to the back up circuit, so that a power supply circuit is intercepted by the back up circuit.
However, among the above-described methods, the methods of (a) and (b) are not practical, because the number of components is increased, a space is increased, and the cost is increased. Further, as the method of (c), for example, a method disclosed in JP-A-2000-152691 (Patent Document 1) has been known. However, this method cannot provide a direct problem resolving means, and it is not possible to determine whether the method is sufficient for ON failure prevention. Finally, according to the method of (d), a monitoring and controlling function needs to be provided in the exterior and similar to the methods of (a) and (b), since the number of components is increased, an apparatus space is increased, and the cost is increased, the method of (d) is also not preferable.
Further, there is another method in which a fuse and a relay (contact-type switch) are assembled for high side switching. According to this method, even when an ON failure occurs in the relay, the fuse independently functions. Therefore, an excess current protecting function is not damaged, and there is no problem in terms of the reliability.
However, the switching structure including a fuse and a relay needs a large space for installation and generates a lot of heat emission. However, when the switching element is replaced by a semiconductor element, such as an FET to solve the space and heat problem, an ON failure of the FET becomes an obstacle. In replacing the switching element by the FET, an ON failure preventing means is essential, but an effective method does not exist in the present time, as described above.
As described above, in the power supply circuit where the semiconductor element, such as an FET, is used as a switching unit according to the related art, when an ON failure in the semiconductor occurs, there is no means to turn off the semiconductor so that a current through the semiconductor continues to flow with no controlling.
Accordingly, a new method is proposed to solve the ON failure problem, wherein once a phenomenon that leads to the ON failure is detected while the semiconductor has capability to turn itself off, the semiconductor can be shut down before it reaches to the ON failure.